This invention relates to an ovenable paperboard to be molded, for instance, into trays or similar food containers.
Referring generally to conventional ovenable paperboards, they are made of sized paperboard, derived from bleached sulfate pulp, on which polyethylene terephthalate (hereinafter noted as PET) or other heat-resistant polymer is extrusion-laminated. Paperboards thus prepared are molded into trays or light containers for food, in which frozen food is packed, and sold in public. Consumers who have bought them are allowed to put trays, as they are or without need of repacking to oven heatable containers, into an electrical oven or a microwave irradiation-type cooking range where frozen foods are thawed and heat-cooked.
In the process as mentioned above, if an oven uses a different heating system from microwave, such as the type of electricity, the defects of darkening discoloration, emission of odor and brittling due to scorching exposure to heat effect take place conspicuously at flange parts of containers as compared with portions thereof covered with the loaded food. These defects give unfavorable appearance, unappetizing feeling and inconvenience in subsequent hand-handling ascribed to brittled flange. The term "flange" will be understood with reference to the specification of U.S. Pat. No. 4,147,836 and its accompanying drawings.
An object of this invention is to provide an ovenable paperboard with so improved heat resistance as to be free from conspicuous scorch at flange parts as mentioned above.
Another object of this invention is to provide an ovenable paperboard where the extrusion-laminated plastic layer is excellently bonded with the substrate paperboard.
A further object of this invention is to provide a process for manufacture of ovenable paperboards having thereon a pigmented adhesive layer, excelling in heat-resistance and adhesivity, opon which an unpigmented natural plastic layer is applied by extrusion lamination.